This invention relates to the production of paper which is strengthened by starch.
It is standard practice to make paper on a paper-making machine by providing a cellulosic thin stock suspension, flocculating the suspension by adding a solution of polymeric retention aid and thereby forming a flocculated suspension, draining the flocculated suspension through a moving screen to form a wet sheet, and carrying the sheet through a heated drying zone and thereby forming a dry sheet. The retention aid can be dissolved cationic starch but is often a synthetic polymeric material. Although the use of polymer of rather low molecular weight can give some improvement in retention, the polymer is preferably of high or very high molecular weight, generally having intrinsic viscosity above 4 dl/g.
A common alternative to this process involves shearing the flocculated suspension so as to degrade the flocs and then adding an aqueous suspension of micro-particulate anionic material and thereby reflocculating the suspension, and then draining the reflocculated suspension through the screen. Such processes using cationic starch and colloidal silica are described in U.S. Pat. No. 4,388,150 and processes using cationic synthetic polymer and bentonite are described in EP-A-235,893. Processes in which size is added after the flocculation with the cationic polymer are described in EP-A-499,448. Processes using other polymers and suspensions suitable for these are described in WO95/02088.
The cellulosic thin stock is often formed in part from recycled paper which may include soluble starch (cationic or anionic or non-ionic) and so the thin stock, and the final sheet, often includes soluble starch. For instance the dry sheet may contain as much as 1% starch derived from recycled paper. It is, however, often desired to add starch to the thin stock.
Thus, water soluble cationic starch may be added as part or all of the solution of polymeric retention aid (see for instance U.S. Pat. No. 4,388,150). The amount required for this purpose is usually not more than about 0.3% (dry weight starch based on the dry weight of paper).
It is often desired to add starch in order to strengthen the paper. For instance it is particularly desirable to include significant amounts of starch in fluting medium and liner board. These materials are usually substantially unfilled and increasing their strength makes them more suitable for use as packaging materials. It is also desirable to include significant amounts of starch in filled sheets as the inclusion of significant amounts of filler would otherwise tend to reduce the strength of the sheet.
In order to maximise strength, it is desirable to include starch in amounts of as much as 5 or 10% or even higher, but attempting to achieve this tends to make the process less efficient as regards energy consumption and/or rate of production, or can incur the risk of unacceptable increase in the chemical oxygen demand of the effluent from the process, because of increased starch in the effluent.
Various grades of starch are conveniently commercially available and include grades which are usually insoluble in the cellulosic suspension. They can be used either unmodified or chemically modified. Generally the starch is pre-solubilised at high temperature to render the starch soluble in the cellulosic suspension.
In this specification when we say a starch is insoluble we mean that it is insoluble in the cellulosic suspension and remains substantially undissolved in the cellulosic suspension. When we say a starch is soluble we mean it is soluble in the cellulosic suspension.
Soluble cationic starch is reasonably substantive to the cellulosic fibres in amounts up to about 1 to 1.5% by weight of the starch, based on the dry weight of the paper. If the amount of cationic starch in the suspension is increased significantly above this, there may be little or no increase in the amount of starch which is retained in the paper and, instead, there is merely an increase in the amount of soluble cationic starch which is in the white water which drains through the screen. This is undesirable since it has to be removed before discharge as effluent, because of the high chemical oxygen demand that it may create in the effluent from the mill.
The soluble cationic starch can be made by chemical modification of starch or merely by cooking raw starch and adding a low molecular weight cationic polymer before, during or after the cooking. Suitable low molecular weight cationic polymers have intrinsic viscosity below 1 dl/g. Examples of such systems are in CA 787,294 and U.S Pat. No. 3,930,877.
In practice, when starch is being used as a strengthening aid it is usually necessary also to include a polymeric retention aid, and there have been various publications about adding combinations of materials. For instance in Tappi June 1976, 59, 6, pages 120 to 122 the performance of various dual polymer systems is examined, including the performance of a blend of soluble cationic starch and hydrolysed polyacrylamide. In CA 1,232,713 up to 1.5% soluble cationic starch is applied in combination with polyethylene oxide or cationic, non-ionic or anionic polyacrylamide retention aid having molecular weight above 1 million.
In Tappi Journal, February 1984, pages 104 to 108, the effects of addition of various starches are examined. It is indicated that starch is generally included in the paper at the size press. It is stated that cationic starch at 1% by weight improves drainage and retention but gives adverse effects at levels above this. It is stated that anionic starches can have either a positive or negative effect on drainage depending on the furnish.
In normal commercial practice it is found that if the amount of cationic starch is increased above about 1 or 1.5% there is increased risk that the cationic starch will interfere with the effectiveness of the polymeric retention aid. As a result retention and drainage may deteriorate with the result that the machine has to operate more slowly or product quality deteriorates.
When it is desired to include a greater amount of starch than 1 to 1.5%, the usual technique involves applying an unmodified starch solution on a size press at the end of the paper-making machine, i.e., after partial or complete drying of the sheet. The application of a solution of starch at this point can result in high pick-up (for instance up to 7 or 10% is common). However it can result in the starch being concentrated more on the surface than in the centre of the sheet and it has the particular disadvantage that it necessitates redrying of the sheet, thus wasting heat energy and/or slowing down the process. It would therefore be desirable to be able to achieve these or higher levels of starch without providing unacceptable levels of soluble starch in the white water and without having to redry the sheet.
Another known method for providing significant loadings of starch in the paper involves applying a spray or a foam containing undissolved starch particles on to the wet sheet before it is carried through the driers, followed by cooking the starch during drying. This process also has the disadvantage of tending to produce a higher concentration of starch on the surface than in the centre of the sheet. However its particular disadvantage is that it is very difficult to achieve uniform application of the starch by spraying or foam application for prolonged periods because of the tendency of the starch composition to cause blockages in the spray or foam applicators.
Attempts to include cold-water insoluble particulate starch in the suspension before drainage have been proposed in the literature but have not achieved success. For instance Fowler reviewed the general techniques of adding starch in Paper 1978 pages 74 and 93. He discussed the techniques mentioned above and also stated that if raw uncooked starch is added to the suspension followed by the addition of retention aid only minimal retention of starch can be achieved. He proposed that better retention is achieved if the starch is slurried with bentonite and added to the suspension prior to the retention aid, and he also proposed that retention can be increased further by including in the slurry a polymer having a charge opposite to the charge of the retention aid.
In U.S. Pat. No. 4,347,100 Brucato describes that mechanical and thermomechanical pulping processes can be improved by adding an anionic surfactant or an anionic polymer during the pulping process. He states that the addition of a cationic polymer causes reaction with the anionic polymer and the formation of a gum-like precipitate which contributes to strength, and he recommends the addition of cationic polymer in a stoichiometric amount based on the anionic polymer. He describes a titration technique for obtaining the desired stoichiometric amount. He also proposes that optimum strength can be achieved by including ungelatinised starch which is gelatinised during subsequent heat drying.
He states that the reaction of the cationic and anionic polymers to produce a gum-like precipitate carries the starch particles and retains the starch in the wood fibres. He says that the furnish is then supplied to the paper-making machine where it is formed into a sheet and heat dried. This suggests that the starch is being added to the pulp or to the thick stock. In all the examples the pulp had a consistency of 2.3% but Brucato suggests higher consistencies are desirable. The strengths are all measured on hand sheets. He gives no information about whether the process could be conducted on a paper making machine, nor how this could be done, nor the extent of retention of starch that can be achieved.
Brucato describes in U.S. Pat. No. 4,609,432 another method of obtaining strengthened paper, this time using two different cellulosic suspensions. 90 to 98% of the fibre weight is provided by a first cellulosic suspension, usually of refined fibres, and 2 to 10% of the fibre weight is provided by adding to this first suspension a second cellulosic suspension which contains a heat-sensitive bonding agent (such as uncooked starch) for bonding the fibres and a polymer for adhering the bonding agent to the fibres of the second suspension. For instance the second suspension can contain the second cellulosic fibres together with 20 to 200% uncooked dry starch and 0.01 to 0.1% cationic polymer. The cationic polymer is said to coat the starch particles and adhere them to the fibres of the second suspension. A typical process uses a first suspension containing 95% of the total fibres and a second suspension containing 5% of the fibres, 0.012% polyethylene imine and 20% starch. A hand sheet was formed from this and was then dried and it appears that the starch is activated during the drying. Again there is no indication about how to conduct the process on a machine nor about retention.
Brucato quotes the same list of cationic polymers in both patents, namely polyethylene imines (which are preferred in U.S. Pat. No. 4,609,432), polyamide polyamine resins, urea formaldehyde resins, melamine formaldehyde resins and polyacrylamides. It seems that Brucato wants to use low molecular weight polymers only since all the classes of polymers he mentions except for the polyacrylamides inevitably have very low molecular weight and the polyacrylamide he exemplifies is Separan CP7, a trade mark of Dow Chemical Co., and we believe that this material also has a relatively low molecular weight, of about 1 million.
There is no suggestion in either of the Brucato patents that any additional retention aid should be used. For instance the highest dosage which is exemplified is around 0.002% based on total fibre weight.
The Brucato methods therefore do not result in the production of a flocculated or reflocculated suspension of the type that is attainable by the use of high molecular weight synthetic polymers or cationic starch optionally followed by anionic microparticulate material.
It is desirable to strengthen substantially unfilled sheets of paper (including paper board) that is to be used as packaging, but there is also a particular need to include starch as a strengthening aid in sheets which are highly filled, since the use of a large amount of filler tends to weaken the sheet. The filler can be preflocculated before addition to the cellulosic suspension. Although this has some advantages, it can cause particular weakening of the sheet. It is therefore known to include water-soluble starch in the pre-flocculated filler composition, but this causes difficulties in handling the flocculated suspension.
In GB 2,223,038 filler is included in a cellulosic suspension by adding a slurry of filler, insoluble starch particles and flocculating agent. The exemplified flocculating agents are anionic or non-ionic, although cationic flocculating agents are mentioned. Those mentioned have low or moderate molecular weight. There is no suggestion to include a high molecular weight cationic flocculating agent. Suspending agents such as a gum, a synthetic organic polymer, or a swelling clay (eg, bentonite), can be included and preferably the suspending agent is chosen so as to reduce the net charge in the composition close to zero. The aim is to flocculate the filler-containing suspension. The resulting flocculated suspension will contain the starch particles trapped in the filler flocs and is added to the cellulosic suspension which is then drained and heated, with consequential cooking of the starch.
Accordingly, none of these detailed methods provide any practical solution to the problem of providing a convenient technique which uses readily available starch and which does not result in undesirable contamination of effluent and which is capable of giving very high pick-up of starch in the paper and which does not involve the problems of size press application or spray or foam application on to the wet sheet.
So far as we are aware, the proposals of Fowler, Brucato and in GB 2,223,038 have not resulted in satisfactory processes for producing sheets containing a large amount of starch as a result of incorporating all the starch in the suspension before drainage. Accordingly, the problem remains that if large amounts of starch are to be incorporated then they have to be added to the wet sheet by spraying or foam or at the size press, and there remains an urgent need to find a way of incorporating starch in the thin stock so as to allow efficient and environmentally acceptable production of paper having a high starch content.
Support for our belief that such a process is not known arises from the fact that in Nordic Pulp and Paper Research Journal Number 4 1994 pages 237 to 241 it is stated that since starch has granular form with diameter of about 1 to 40 .mu.m the retention of the starch granules is very low when added directly to paper stock without dissolution or swelling in water. According to the proposals in this article it is possible to include high amounts of starch in laboratory handsheets by including in the cellulosic suspension starch having a particular flake form and which has been made by precipitation in mineral salts and processing the precipitate. It is commercially undesirable to have to undergo this particular process and it would be much more convenient to be able to obtain high starch levels in paper made on a conventional paper-making machine using conventional granular starches and without incurring significant effluent problems due to excessive drainage of starch through the screen.